Acids & Bases

Question 1

What are the 3 physical properties of acids?

Answer 1

1. Acids taste sour
2. Dilute acids are irritants and will result in rashes and blisters
3. Acids turn blue litmus paper red

Question 2

What makes a compound/substance an acid?

Answer 2

If it dissociates H+ ions when dissolved in water

Question 3

Acid + Metal ——> ?

Answer 3

Salt + H2

Question 4

Acid + Base ——> ?

Answer 4

Salt + H2O

Question 5

Acid + Metal carbonate ——> ?

Answer 5

Salt + H2O + CO2

Question 6

What type of reaction is this? Acid + Base ——> Salt + H2O

Answer 6

Neutralisation reaction

Question 7

How do you differentiate between strong acids and weak acids? Give an example of a strong and weak acid.

Answer 7

Strong acids have the ability to dissociate fully in water to produce a high concentration of H+ ions. Weak acids only have the ability to dissociate partially in water to produce a low concentration of H+ ions. Hydrochloric acid is a strong acid while hydrofluoric acid is a weak acid.

Question 8

Explain concentration of an acid.

Answer 8

• Concentration of acid is dependent on how diluted the solution is (the dilution factor)
• Concentration refers to number of acid molecules present per water molecule
• Usually expressed in mol/dm-3

Question 9

What is basicity of an acid?

Answer 9

The basicity of an acid depends on the acid’s chemical formula. Basicity refers to number of H+ ions produced per molecule. For example, HCl produces 1 H+ ion and is monobasic, H2SO4 produces 2 H+ ions and is dibasic, H3PO4 produces 3 H+ ions and is tribasic

Question 10

Can a strong acid have a low concentration of H+ ions?

Answer 10

Yes, use a strong acid like nitric acid and add a large amount of water to it.

Question 11

If we dissolve 2 strong acids, nitric acid and sulfuric acid, in water, which one will produce a higher concentration of H+ ions?

Answer 11

Even though both strong acids will dissociate fully in water, sulfuric acid would produce a higher concentration of H+ ions. Sulfuric acid is dibasic, producing 2 H+ ions per molecule while nitric acid is monobasic, producing only 1 H+ ion per molecule.

Question 12

What is a base?

Answer 12

A base is defined as a substance which reacts with an acid to form a salt and water only. All metal oxides and hydroxides are bases.

Question 13

What are alkalis?

Answer 13

Alkalis are a special group of bases that are soluble in water. Hence, alkalis dissociate either fully or partially in water to produce OH- ions, giving rise to its alkaline properties. In other words, alkalis are a subset of base.

Question 14

The oxides and hydroxides of metals in which 2 groups of the periodic table form alkalis?

Answer 14

Group l and Group II

Question 15

Name the 4 physical properties of alkalis

Answer 15

1. Alkalis taste bitter
2. Dilute alkalis feel slippery and soapy to touch
3. Concentrated alkalis are caustic, causing chemical burns
4. Alkalis turn red litmus paper blue

Question 16

What makes a compound/substance an alkali?

Answer 16

A compound is considered an alkali when it dissociates OH- ions when it dissolves in water.

Question 17

What is a weak alkali?

Answer 17

A weak alkali is one that only dissociates partially in water to produce a low concentration of OH- ions.

Question 18

What must you take note of when writing the dissociation reaction for a weak alkali or acid? And why?

Answer 18

• Must write the reversible sign ⇌. Since it is a weak alkali, some of the ions may turn back to the atom.
• A commonly used weak alkali is aqueous ammonia. Dissociation reaction for ammonia is NH3 + H2O ⇌ NH₄⁺ + OH-. Some NH₄⁺ ions turn back to NH3, resulting in low concentration of OH- ions produced.

Question 19

What are the 4 uses of alkalis?

Answer 19

1. Toothpaste to neutralise acid on teeth
2. Calcium hydroxide used to neutralise acidity in soil
3. Magnesium hydroxide in indigestion tablets, aka antacid pills
4. Sodium hydroxide in floor & cleaners

Question 20

Alkali + Ammonium salt ——> ?

Answer 20

Salt + H2O + NH3 (g)

Question 21

Why must litmus paper be moist when testing for ammonia gas?

Answer 21

Moist litmus paper must be used in order for the ammonia gas to dissolve in water and dissociate to form OH- ions.

Question 22

If HCl (aq) is hydrochloric acid, does this mean HCl (l) which is hydrogen chloride, an acid as well?

Answer 22

No. An acid is only considered as an acid after it has dissociated in water and H+ ions are formed. This is the same for alkali. An alkali is only an alkali after it has dissociated in water and formed OH- ions.

Question 23

What must you remember when writing dissociation reaction for weak alkalis?

Answer 23

Must add water. For example, write NH3 + H2O ⇌ NH₄⁺ + OH- and not NH3 ⇌ NH₄⁺ + OH-.

Question 24

The pH scale ranges from 0 to 14. pH 7 is considered neutral. pH 14 is considered very _____ and ph 1 is considered very _____.

Answer 24

Alkaline; acidic

Question 25

The indicator methyl orange is _____ when acidic, _____ when neutral and _____ alkaline.

Answer 25

Red; orange; yellow

Question 26

The indicator screened methyl orange is _____ when acidic, _____ when neutral and _____ alkaline.

Answer 26

Purple; grey; green

Question 27

The indicator phenolphthalein is _____ when acidic, _____ when neutral and _____ alkaline.

Answer 27

Colourless; pale pink; pink

Question 28

The indicator bromothymol blue is _____ when acidic, _____ when neutral and _____ alkaline.

Answer 28

Yellow; green; blue